Assuming no electrical output, whatever energy enters the amplifier would be given off as heat. Is your amplifier, while on but idle, giving off as much heat as one or two 100-watt light bulbs? I would suspect that the idle power consumption would be closer to 5 or 10 watts per channel.
As to the perceived sound changing in the first 30 minutes of use, likely yes. Whether one can accurately and definitively assign the cause to amplifier warm-up, not likely without controlled testing. The variability in other components, some in the electromotive physical devices that are your loudspeakers, and the continually variable and adjusting perceptions of one's own psycho-acoustic listening system (ears, brain, expectations, thoughts, emotions, ever-changing placement and orientation of the receptors, etc.) not allowing for consistent 'measurement' of results means that, unless your amplifier is malfunctioning, one is better leaving it to the test lab to tell whether the amplifier is constant/consistent or non-constant/inconsistent in its behavior.